Steerable drilling systems are increasingly used to controllably drill a deviated borehole from a straight section of a wellbore. In a simplified application, the wellbore is a straight vertical hole, and the drilling operator desires to drill a deviated borehole off the straight wellbore in order to thereafter drill substantially horizontally in an oil bearing formation. Steerable drilling systems conventionally utilize a downhole motor (mud motor) powered by drilling fluid (mud) pumped from the surface to rotate a bit. The motor and bit are supported from a drill string that extends to the well surface. The motor rotates the bit with a drive linkage extending through a bent sub or bent housing positioned between the power section of the motor and the drill bit. Those skilled in the art recognize that the bent sub may actually comprise more than one bend to obtain a net effect which is hereafter referred to for simplicity as a "bend" and associated "bend angle."
To steer the bit, the drilling operator conventionally holds the drill string from rotation and powers the motor to rotate the bit while the motor housing is advanced (slides) along the borehole during penetration. During this sliding operation, the bend directs the bit away from the axis of the borehole to provide a slightly curved borehole section, with the curve achieving the desired deviation or build angle. When a straight or tangent section of the deviated borehole is desired, the drill string and thus the motor housing are rotated, which generally causes a slightly larger bore to be drilled along a straight path tangent to the curved section. U.S. Pat. No. 4,667,751, now RE 33,751, is exemplary of the prior art relating to deviated borehole drilling. Most operators recognize that the rate of penetration (ROP) of the bit drilling through the formation is significantly less when the motor housing is not rotated, and accordingly sliding of the motor with no motor rotation is conventionally limited to operations required to obtain the desired deviation or build, thereby obtaining an overall acceptable build rate when drilling the deviated borehole. Accordingly, the deviated borehole typically consists of two or more relatively short length curved borehole sections, and one or more relatively long tangent sections each extending between two curved sections.
Downhole mud motors are conventionally stabilized at two or more locations along the motor housing, as disclosed in U.S. Pat. No. 5,513,714, and WO 95/25872. The bottom hole assembly (BHA) used in steerable systems commonly employs two or three stabilizers on the motor to give directional control and to improve hole quality. Also, selective positioning of stabilizers on the motor produces known contact points with the wellbore to assist in building the curve at a predetermined build rate.
While stabilizers are thus accepted components of steerable BHAs, the use of such stabilizers causes problems when in the steering mode, i.e., when only the bit is rotated and the motor slides in the hole while the drill string and motor housing are not rotated to drill a curved borehole section. Motor stabilizers provide discrete contact points with the wellbore, thereby making sliding of the BHA difficult while simultaneously maintaining the desired WOB. Accordingly, drilling operators have attempted to avoid the problems caused by the stabilizers by running the BHA "slick," i.e., with no stabilizers on the motor housing. Directional control may be sacrificed, however, because the unstabilized motor can more easily shift radially when drilling, thereby altering the drilling trajectory.
Bits used in steerable assemblies commonly employ fixed PDC cutters on the bit face. The bit gauge length is the axial length of the sleeve extending from the bit face, and typically is formed from a high wear resistant material. Drilling operations conventionally use a bit with a short gauge length. A short bit gauge length is desired since, when in the steering mode, the side cutting ability of the bit required to initiate a deviation is adversely affected by the bit gauge length. Along gauge on a bit is commonly used in straight hole drilling to avoid or minimize any build, and accordingly is considered contrary to the objective of a steerable system. A long gauge bit is considered by some to be functionally similar to a conventional bit and a "piggyback" or "tandem" stabilizer immediately above the bit. This piggyback arrangement has been attempted in a steerable BHA, and has been widely discarded since the BHA has little or no ability to deviate the borehole trajectory. The accepted view has thus been that the use of a long gauge bit, or a piggyback stabilizer immediately above a conventional short gauge bit, in a steerable BHA results in the loss of the drilling operator's ability to quickly change direction, i.e., they do not allow the BHA to steer or steering is very limited and unpredictable. The use of PDC bits with a double or "tandem" gauge section for steerable motor applications is nevertheless disclosed in SPE 39308 entitled "Development and Successful Application of Unique Steerable PDC Bits."
Most steerable BHAs are driven by a positive displacement motor (PDM), and most commonly by a Moineau motor which utilizes a spiraling rotor which is driven by fluid pressure passing between the motor and stator. PDMs are capable of producing high torque, low speed drilling that is generally desirable for steerable applications. Some operators have utilized steerable BHAs driven by a turbine-type motor, which is also referred to as a turbodrill. A turbodrill operates under a concept of fluid slippage past the turbine vanes, and thus operates at a much lower torque and a much higher rotary speed than a PDM. Most formations drilled by PDMs cannot be economically drilled by turbodrills, and the use of turbodrills to drill curved boreholes is very limited. Nevertheless, turbodrills have been used in some steerable applications, as evidenced by the article "Steerable Turbodrilling Setting New ROP Records," OFFSHORE, August 1997, pp. 40 and 42. The action of the PDC bit powered by a PDM is also substantially different than the action of a PDC bit powered by a turbodrill because the turbodrill rotates the bit at a much higher speed and a much lower torque.
Turbodrills require a significant pressure drop across the motor to rotate the bit, which inherently limits the applications in which turbodrills can practically be used. To increase the torque in the turbodrill, the power section of the motor has to be made longer. Power sections of conventional turbodrills are often 30 feet or more in length, and increasing the length of the turbodrill power section is both costly and adversely affects the ability of the turbodrill to be used in steerable applications.
Those skilled in the art have long sought improvements in the performance of a steerable BHA which will result in a higher ROP, particularly if a higher ROP can be obtained with better hole quality and without adversely affecting the ability of the BHA to reliably steer the bit. Such improvements in the BHA and in the method of operating the BHA would result in considerable savings in the time and money utilized to drill a well, particularly if the BHA can be used to penetrate farther into the formation before the BHA is retrieved to the surface for altering the BHA or for replacing the bit. By improving the quality of both the curved borehole sections and the straight borehole sections of a deviated borehole, the time and money required for inserting a casing in the well and then cementing the casing in place are reduced. The long standing goal of an improved steerable BHA and method of drilling a deviated borehole has thus been to save both time and money in the production of hydrocarbons.